Dir. Office
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H. Grunder
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J. Boyce
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AccD File
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C. Piller
P. Piot
T. Powers
J. Preble
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Q.S. Shu
M. Tiefenback
R. Ursic
D.X. Wang
Test Lab Library / MS 85A
J. Benesch
M. Bickley
B. Dunham
A. Grippo
A. Hofler
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B. Legg
H. Liu
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M. Spata
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J. Tang
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R. Sundelin
MS 28H
C. Ficklen
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L. Cardman
R. Carlini
K. de Jager
J. Grames
B. Mecking
S. Nanda
A. Saha
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P. Brindza
J. O'Meara
Library (5 copies)
(MS 28E)
TITLE: Occupational and Environment Aspects of the Radiation Control Provisions at the Jefferson Lab / TN#: JLAB TN
97–017 - May 1997
REV A July 1997
REV B October 1997
AUTHOR(S): P. Degtiarenko, R. May, S. Schwahn, G. Stapleton
KEYWORD(S):
Accelerator Physics Diagnostics Magnets
Arc Environment, QA Nuclear Physics
Beam Dynamics Experimental Equipment RF
Beam Transport Extraction Safety and Health Physics
BSY Failure Mode/Tests Schedule
Civil Construction Free Electron Laser (FEL) SRF
Commissioning Front End Test Results
Controls Injector Vacuum
Cost Installation Other
Cryogenics Integration
DC Power Linac
ABSTRACT:
Appendix 1 added to document.
STANDARD DISTRIBUTION
Jefferson Lab Technical Notes are informal memos intended for rapid, internal communication of work in progress. Of necessity, these notes are limited in their completeness and have not undergone a pre-publications review.
JLAB TN 97–017
May 1997
REV A July 1997
REV B October 1997
Occupational and Environment Aspects of the Radiation Control Provisions at the Jefferson Lab
P. Degtiarenko, R. May, S. Schwahn, and G. Stapleton
TABLE OF CONTENTS
1.0 INTRODUCTION
1.1 SUMMARY
1.2 DISCUSSION OF ACCELERATOR RADIATION
1.3 HISTORICAL DEVELOPMENT OF RADIATION PROTECTION AT THE JEFFERSON LAB
2.0 DIRECT OR PROMPT RADIATION
2.1 BEAM SAFETY
2.2 OCCUPATIONAL EXPOSURE CONTROL
2.3 MEMBERS OF THE PUBLIC (OFF-SITE)
3.0 INDUCED RADIOACTIVITY
3.1 OCCUPATIONAL EXPOSURE
3.2 MEMBERS OF THE PUBLIC (OFF-SITE) AND ENVIRONMENTAL
3.3 STORAGE AND RELEASE OF ACTIVATED MATERIALS
4.0 OZONE PRODUCTION
5.0 CONCLUSIONS
6.0 ACKNOWLEDGMENTS
7.0 REFERENCES
APPENDIX 1 52
ii
1.0 INTRODUCTION
1.1 Summary
The purpose of this tech note is to provide a detailed account of the provisions made for the control of radiation from the CEBAF accelerator and experimental halls at the Thomas Jefferson National Accelerator Facility with particular emphasis on those aspects which are of environmental concern. Although the radiation protection provisions were originally predicated on an accelerator capable of accelerating electron beam currents of up to 200mA at energies up to 4.0 GeV with the resulting maximum beam power of 800 kW, it is considered that the radiation control provisions made would, in the main, be entirely adequate for an increase in beam energy to 8 GeV with the proviso that the total power of the accelerated beam would not exceed the present DOE approved safety envelope of 1 MW.
This tech note begins with a general discussion of radiation control aspects of accelerators in order to emphasize their unique character, especially for readers with limited experience with accelerators. The note then references and reviews earlier work on radiation protection at CEBAF and, where appropriate, updates or corrects earlier conclusions in the light of more recent information obtained, including the use of more sophisticated computer modeling techniques. The note concludes by summarizing the arguments that lead to the conclusion that the facility can be safely operated at up to 8 GeV without compromise to any radiation control practice or any relevant environmental control regulation.
1.2 Discussion of accelerator radiation
Accelerator particle beams can only be sustained by a complex optical system of pulsed or DC powered electro magnets, RF fields and voltage gradients. The total stored energy in the CEBAF accelerator is rather small, only a few joules, so that as soon as the beam is terminated, the source of prompt radiation is removed.
Even though sensitive instruments coupled with interlocks are typically installed to rapidly detect, prevent or mitigate excessive beam loss, designers have never taken this to mean that accelerators can or should be built without any shielding.
Prompt radiation arises principally from beam loss that is a routinely expected consequence of accelerator operations:
(a) All accelerators will give rise to prompt radiation, during operation, due to inefficiencies in confining the beam to the design orbit. The physics behind this loss mechanism is complex and depends on the type of accelerator and species of charged particle accelerated. The mechanisms include gas scattering and gas interactions, quantum effects such as synchrotron radiation production, space charge interactions, beam break up and wake field effects, and emittance dilution due to intrinsic errors in the accelerating structure and magnet optics. Additional complexity results from the use of primary beams to produce secondary beams of different sub-atomic particles.
(b) Beam loss also results from deliberate, but small, spills for the purpose of conducting beam-physics studies to improve or upgrade the accelerator performance or to study and verify the integrity of the various installed safety systems.
(c) Beam loss can also occur due to occasional failure of beamline optical elements or other components of the accelerator system.
(d) Beam loss can also occur due to mistuning of the apparatus by the operators.
(e) Beam loss also occurs by design in order to use the beams for research experiments or other applications.
The design of shielding for accelerators is usually based on an estimate of normal beam loss from routine operations, such as described above, resulting in a source term of some fraction of the total beam power, typically 0.1% or as low as 0.01% in a few cases. For major beam loss regions, such as targets, collimators and beam dumps, the shielding will be designed typically for 100% of beam power. Using the above rationale, the effect of an accidental full beam loss can be simply quantified where the shielding is specified for normal losses at, for example, 0.1% or 0.01% of full beam power. Since shielding might, for example, be designed to ensure average radiation levels in occupied areas of perhaps, 0.1 mrem/hour normalized to a 2000 hour working year, by contrast simple scaling from 0.1 mrem/h by at most a 10000-fold increase - i.e., for the case of the thinnest shielding - shows that the dose rate even then cannot exceed 1.0 rem/hour. This illustration is not intended to provide any prescription for accelerator shielding which normally requires an in depth analysis of all the operational considerations including occupational and work practices.
In addition to specifying adequate shielding, diagnostic instruments usually alert the operator to the various beam-loss conditions and/or interrupt the beam if necessary. Personnel safety systems are provided in parallel with the diagnostic or machine protection instruments to ensure that the Safety Envelope is not compromised. All these systems together provide adequate safety to personnel by preventing occupancy or access during beam-on conditions and likewise preventing errant beams from entering occupied areas.
Machine protection devices used to safeguard the accelerator from costly damage are generally not included in any scheme of personnel safety. The reason for this is not that such systems are inherently less well engineered than those on personnel safety systems, it is because such systems are essentially “open” to the accelerator operations crew to adjust or use in the most efficient way. Nevertheless in the context of an overall integrated approach, such machine protection devices have an important role in reducing the number of challenges to the personnel safety system and hence, enhancing the overall reliability of the personnel devices. Furthermore, should any machine protection system be reconfigured so as to make the system “closed” to all except certified staff, then such systems might then be included in the personnel safety system. As a general observation it can be said that accelerators are exceedingly expensive pieces of equipment so that any device designed to protect them can usually be counted on to work rather well.
None of the techniques or systems discussed are totally free from a possibility of failure; a person could get hurt and become unconscious while working inside a beam enclosure and be missed during a search sweep; door switches and their connecting conduits can be damaged and fail to function; shielding such as earth berms can be excavated or shielding blocks can be moved without proper authorization; and an as-built shielding configuration can also be in error. Traditionally, the systematic approach to safety at research accelerators has not relied on any single element for the safety of personnel or even the accelerator components. In both cases (personnel protection and machine protection), the level of protection results from an integration of engineered systems, that include shielding, strict administrative procedures, and continuous training of personnel. These various elements provide the “defense-in-depth” approach that has been successful in accelerator, nuclear power and other industrial safety systems. Failure of one barrier or element will not result in complete loss of protection. The combined system, through redundancy and diversity, minimizes the risk due to total failure.
Accelerator beams, when allowed to strike an object can make it become radioactive. The extent to which “induced” radioactivity is produced depends on the accelerated particle - its type, energy and intensity. In the case of proton or heavy ion accelerators, the coulomb barrier or a high negative Q of the reaction together with the kinematics of momentum transfer could prevent any nuclear reaction occurring below a given energy threshold for the bombarding particle (Friedlander et al. 1964). In the case of CEBAF which accelerates electrons, the nuclear processes are somewhat different. Electrons are leptons and do not interact with the strong nuclear force, but can still scatter from a nuclear particle with sufficient momentum transfer to result in a nuclear reaction. Such reactions occur with considerably lower probability than if the bombarding particle were a neutron or proton. There is also a further way in which electrons interact with the nucleus and that is via photons. The most important mode of energy loss by electron beams above a few MeV in energy is by the bremsstrahlung process in which an electron is deflected by the Coulomb field of the atomic nucleus to produce a high energy photon. The cross section for this radiative process is well defined and understood and is most commonly tabulated as the radiation length for a given material; at CEBAF energies it is practically independent of energy. A further process in which the photon converts to an e- e+ pair operates with similar probabilities. Radiation lengths for typical materials are: liq H2 = 865 cm, liq D = 757 cm, C = 18.8 cm, Al = 8.9 cm, Fe = 1.76 cm. This repeated bremsstrahlung/pair production process has the effect of rapidly multiplying the numbers of charged particles present and hence the energy transfer to the medium in which the shower is created; it also produces an intense and highly penetrating photon field. The photon may also undergo a nuclear reaction either by a resonance process favored by certain photon energies or by behaving as a sort of hadron (vector meson) (Schopper 1990). These photon nucleus reactions occur with higher probability than for an electron interaction.
However, all these nuclear reactions do have attributes in common and, of particular interest to the Health Physicist, is that the resultant radionuclides, whether generated directly by the primary accelerated particle or by any subsequent particles produced in the interactions of the accelerated particle, are likely to be to the neutron deficient side of the nuclear stability line. This means that much of the induced activity will decay by electron capture and positron emission - such radionuclides are generally of rather low radiotoxicity. Neither is it possible for any of the highly toxic, alpha particle emitting, radionuclides associated with the nuclear fission cycle being produced from activation of the normal medium atomic weight material used in accelerator components and construction materials.
In brief, accelerator radiation can be extremely intense but it is very local in extent and can be shielded and controlled. Radioactive material produced by accelerator beams is generally produced within massive structural material and cannot be dispersed (radioactivity in air and water is discussed separately) and the toxicity of the radioactive material is generally low and the half lives of such material are usually rather short. A further discussion of this subject is given by Barbier and also by Sullivan (Barbier 1969a, Sullivan 1992). Some surface radioactivity (dust) can be found at some high energy accelerators but this is generally of small concern because it is mainly due to beryllium-7 which is a very low toxicity radionuclide.
1.3 Historical development of radiation protection at The Jefferson Lab
The following discussion traces the development of the radiation protection provisions for CEBAF from the early conceptual designs to the present and provides a listing of all the relevant documentation. The reader should note that the laboratory’s name was changed from the Continuous Electron Beam Accelerator Facility (CEBAF) to Thomas Jefferson National Accelerator Laboratory (Jefferson Lab) in 1996; however, to avoid confusion in referencing the earlier activities at the laboratory, the name CEBAF is retained where appropriate.